Vacuum device



1964 M. P. HNILICKA, JR., ETAL 3,119,243

VACUUM DEVICE Filed April 4, 1962 INVENTORS.

MILO P HNILICKA, JR.

Md. M

United States Patent Ofilice 3,119,243 Patented Jan. 28, 1964 5,119,243VACUUM DEVICE Milo P. Hnilicka, Ilia, Concord, and John C. Simona, Sin,

Weston, Mass, assignors to National Research Corporation, Cambridge,Mass, a corporation of Massachusetts Filed Apr. 4, H62, er. No. 185,1276 Claims. c1. eZFssi This invention relates to Vacuum facilities andmore particularly to extremely high vacuum facilities for mamtainingpressures below 10* torr.

From a practical viewpoint the current limitation of the ultrahighvacuum art is about 1X 10- torr although a few workers have attainedlower pressures in very small volumes constructed of glass. Thedesirability of providing pressures of at least 2 to 5 decades lowerthan 1() torr in large volumes is of considerable importance since aWide variety of surface phenomena could then be investigated. At 1 l0"torr there is less than one hour of time to study the behavior offreshly cleaned surfaces before a significant portion of the surfacebecomes covered with at least a monolayer of gas. However, at a pressureof torr, for example, this time is extended into days. Knowledge of thebehavior of clean surfaces is not only important as a fundamentalinterest, but also is important with respect to applications to spacevehicles.

Ultimate pressures attainable in vacuum systems depend on the gas loadpresent and on the pumping speed available. When the pumping speedsapproach the maximum permitted by the size of the vacuum system theultimate pressure attainable is then limited by the gas load. The gasload may be minimized in two ways. Much of the gas load results from gasadsorbed on the metallic surfaces or dispersed through the bulk of themetal. The adsorbed gas can be released from the surface in part bybaking the metallic surfaces at an elevated temperature. The rate ofevolution of the dispersed gases, particularly hydrogen, can be greatlyreduced by chilling the walls of the system to cryogenic temperatures.

It is a principal object of the present invention to provide anextremely high vacuum system which will provide pressures of less thanl( torr.

Still another object of the invention is to provide a vacuum systemhaving volumes of useful size which can be maintained at pressures ofless than 10 torr.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the apparatus possessing theconstruction, combination of elements and arrangement of parts which areexemplified in the following detailed disclosure and the scope of theapplication of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing which is a diagrammaticschematic sectional view of a preferred embodiment of the invention.

in a preferred embodiment of the invention, by which the above objectsare accomplished and the limitations of the prior art overcome, there isprovided an inner or working chamber which is to be maintained at apressure of less than 1t)" torr. Associated with the inner chamber isrefrigerating means which serves to maintain a major portion of theinner chamber at a temperature below about 20 K. and preferably at atemperature of about 4 K. to 10 K. Cooling of the inner chamber walls toa temperature of below 20 K. comprises an important feature of thepresent invention since outgiassing, particularly of hydrogen, from thewalls of the inner chamber will be held to a negligible rate.Additionally ambient gases will be pumped at high speed by condensationon the walls of the inner chamber. Surrounding the inner chamber andspaced therefrom is an intermediate chamber. Associated with the wallsof the intermediate chamber is a second refrigerating means which servesto cool the wall of the intermediate chamber to a temperature belowabout K. and preferably to a temperature on the order of 77 K. In thesame manner as with the inner chamber cooling of the walls of theintermediate chamber greatly reduces outgassing from the walls by manyorders of magnitude and removes ambient gases by condensation on theWalls. Adjacent to the forward end of the inner chamber there isprovided means for evacuating gases from the intermediate chamber tomaintain the intermediate chamber at a pressure on the order of 10-torr. Referring again to the inner chamber there is provided in thebackward end of this chamber an opening which communicates with theintermediate chamber. This opening which communicates with theintermediate chamber permits the inner ch mber to be evacuated by thesame pumping means which evacuates the intermediate chamber. Thelocation of the opening in the inner chamber comprises another importantfeature of the present invention. The opening which is in the backwardend of the inner chamber is thus at a point remote from the pumpingmeans which is positioned adjacent to the forward end of the innerchamber. Any backstreaming of gases from or through the pump must travelthe length of the inner chamber through the space defined by the wallsof the inner chamber and the intermediate chamber. Thus the cooledsurfaces of the inner chamber and intermediate chamber will act as acold trap which will condense backstreaming gases and prevent such gasesfrom entering the inner chamber. This serves to maintain a high vacuumwithin the inner chamber. Adjacent the opening of the inner chamberthere is provided a bafile which serves to restrict heat radiation fromthe intermediate chamber walls to the inner chamber. Around theintermediate chamber and spaced therefrom there next is provided anouter chamber. The outer chamber restrains the atmosphere and isseparately evacuated to provide a vacuum tight space around theintermediate chamber. The vacuum tight space formed by the outer chamberserves to reduce the consequence of leaks and provides thermalinsulation for the cold intermediate chamber. The outer chamber andintermediate chamber are sealed together at the forward end of thevacuum facility so as to form an integral unit which can easily beremoved to provide access to the inner chamber. Any gas leaks from theseal connecting the outer chamber and the intermediate chamher willenter the outer chamber only and be evacuated by the pumping systemassociated therewith. The forward end of the vacuum chamber preferablycomprises an outer transition section and an inner tubular section. Thetubular section forms the forward end of the intermediate chamber andalso extends into the intermediate chamber to provide support for theinner chamber which can accommodate numerous electrical andinstrumentation leads. The outer transition section of the end member isconcentric about a portion of the tubular section adjacent the innerchamber. The transition section and the tubular section which form theend member communicate with each other and together form a vacuum tightchamber which is separately evacuated to a low pressure on the order of10* torr. An important feature of the present invention is that the endmember is constructed and arranged to provide a seal with theintermediate chamber whereby any gases originating from the seal willenter the end member and be removed therefrom and hence will not enterthe intermediate chamber. The end section in addition to providingsupport for the inner chamber can, if desired, provide communicationbetween the inner chamber and exterior of the vacuum facility. Thetubular section of the end member is of advantage in that it forms atunnel for experimental purposes such as beam experiments. Associatedwith the walls of the tubular section is refrigerating means whichserves to cool the walls of the tubular section to a temperature belowabout 100 K. and preferably to a temperature on the order of 77 K.Cooling of at least a portion of the walls of the tubular section isimportant in that it serves to reduce outgassing and also reduces heattransfer to the inner chamber which is supported by the tubular section.Positioned within the intermediate chamber are heating means which serveto heat the walls of the inner chamber, the intermediate chamber, andthat portion of the end member which extends within the intermediatechamber to temperatures of up to about 1000" C. to remove adsorbedgases.

Referring now to the drawing there is shown a schematic, diagrammatic,sectional view of one preferred embodiment of the invention. Theapparatus comprises a first wall defining an elongated inner chamber 12which is to be maintained at a pressure below about 10'- torr.Associated with chamber 12 are cooling coils 14 which provide forcirculation of a cooling fluid in heat exchange relationship with wall10. Preferably gaseous helium from a supply 16 is circulated throughcoils 14 to cool a major portion of wall 10 to a temperature below aboutK. and preferably to a temperature of between about 10 K. to 4 K. Asecond wall 18 spaced from a first wall 10 defines an intermediatechamber 20. Associated with chamber 20 are cooling coils 22 whichprovide for circulation of a cooling fluid in heat exchange relationshipwith wall 18. Preferably liquid nitrogen from supply 24 is circulatedthrough coils 22 to cool wall 18 to a temperature below about 100 K.,preferably on the order of 77 K. Chamber 12 preferably communicates withchamber 20 through an opening 26 in the rearward end of chamber 12. Abaffle '28 is preferably positioned in a line of sight adjacent opening26. Baflle 23 serves to restrict the radiation of heat from wall 18directly into the inner chamber 12 but does not prohibit evacuation ofgases from chamber 12. A vacuum pump 30 is positioned adjacent to theforward end of the inner chamber :12 and serves to evacuate gases fromthe intermediate chamber 20 and inner chamber 12 through the opening 26.A third wall 32 spaced from the second wall 18 defines an outer chamber34 which is separately evacuated by pump 36 to a low pressure. The outerchamber 34 and the intermediate chamber 20 and sealed together by flangejoints 3S and 40. Preferably the flanged joints are of the cooled double0 ring design as described in the copending application of Farkass,Serial No. 3,674, filed January 20, 1960, now Patent No. 3,058,232. Theforward end of the vacuum facility comprises an outer transition section42 and an inner tubular section 44 which extends into the intermediatechamber and supports the inner chamber 12. The tubular sectioncooperates with an annular disc 46, which is sealed to the tubularsection by bolt means 37 land sealingly held to flange 38 by springmeans 39, to form the forward end of the intermediate chamber 20. Theouter transition section 42 is concentric about the tutbular section 44and communicates with the interior of the tubular section by means ofopenings 48. Preferably the transition section 42 is sealed to thetubular section 44 by flanges 66, 68 of cooled double 0 ring design. Thetubular section together with the transition section form a vacuum tightchamber 49 which is separately evacuated by pump 50 to a pressure on theorder of 10- torr. The transition section is preferably sealed to flange38 of the intermediate chamber by a cooled double 0 ring flange 52. Anopening 54 is provided between disc 46 and the wall of the transitionsection 42. Gases originating from the seal of flange 52 or from theatmosphere preferentially enter the transition section through opening54 and are then removed by pump 50, thereby preventing the gases fromentering the intermediate chamber 20. The portion of the tubular section44 within the transition section is cooled to a temperature below K. andpreferably on the order of 77 K. by circulating liquid nitrogen from asupply (not shown) through coils 43. A liquid nitrogen cooled baflle 70is provided within the tubular section 44 to restrict heat transfer tothe inner chamber 12. The portion of the tubular section 44 which isexposed to the atmosphere is preferably cooled by circulating waterthrough coils 45. The vacuum pumps 30, 36 and 50 are preferably oildiffusion pumps. Preferably pumps 30 and 50 are provided with cold capsand liquid nitrogen cooled baffles (58, 60) and are preferably backed bysecond diffusion pumps 62 and 64 and by mechanical pumps (not shown).Pump 36 is also preferably backed by a mechanical pump (not shown).

Positioned within the intermediate chamber are heating means 56 whichserve to heat the walls of the inner chamber, the intermediate chamberand that portion of the tubular section which extends within theintermediate chamber to a temperature on the order of 200 C. to 1000 C.Preferably the heating means 56 are radiant heaters. The vacuum facilityis preferably constructed of stainless steel such as 304 stainlesssteel. While other suitable metals or alloys can be used, the mainrequirements are that they provide sufiicient strength to withstand theambient pressures and be capable of being heated to tempenatures of upto 1000 C.

In a preferred embodiment of the invention the vacuum-tight space formedby the outer chamber 34 is filled with a thermal insulation material.Thermal insulating materials such as those described in US. Patent3,018,- 016 to Hnilicka and U.S. Patent 2,900,800 to Loveday aresuitable.

In operation of the vacuum facility described above, chambers 12, 18, 34and 49 will be initially evacuated by mechanical vacuum pumps (notshown). When a pressure of about 10" to 10* torr is attained, diffusionpumps (50, 62), (30, 64) and 36 will be activated. Heaters 56 will alsobe activated to heat the intermediate chamber, the inner chamber, andthat portion of the tubular section 44 which extends into theintermediate chamber, so as to drive off adsorbed gases and liquids.With the diffusion and mechanical pumps, a pressure on the order of 10-to 10* torr can be obtained at the elevated bake-out temperatures. Atthis time the heaters will be turned off and liquid nitrogen from supply24 will be circulated through coils 22 to lower the temperature of theintermediate chamber wall 18 to about 77 K. This temperature issulficiently low to greatly reduce outgassing and condense water vapor,carbon dioxide residual hydrocarbons and the like. In this manner apressure of about 10 torr will be maintained in the intermediate chamber20 and the end member chamber 49. Thereafter gaseous helium from supply16 will be circulated through coils 14 to lower the temperature of theinner chamber wall 10 to about 8 K. This temperature is sufliciently lowso that outgassing will be drastically reduced and all gases excepthydrogen, neon and helium will be pumped at high speed by condensationon the inner chamber wall. The release of hydrogen from steel will bereduced by many orders of magnitude by lowering the steel temperature to77 K. The H evolution rate at steel temperatures of 8 K. is suflicientlylow to be negligible even at the ultrahigh vacuum contemplated. In thismanner a pressure on the order of 10' to 10- torr and lower can beattained in the inner chamber 12.

As the vacuum facility operates, any gas leak from the double 0 ringseal formed by flanges 38 and 40 will be removed from the outer chamber34 by pump 36 which will maintain chamber 34 at a pressure on the orderof 10" to 10* torr. Thus the outer chamber 34 will act as a guard volumeand also provide thermal insulation for the cold intermediate chamber.Backstreaming of vapors from oil diffusion pumps 30 and 64 comprise bothpump fluid vapor and fiuid thermal decomposition products. While thefluid vapors can be removed by condensation on the liquid nitrogen traps58, thermal decomposition products and other gases which may pass backthrough the pump may not be condensable at the liquid nitrogentemperature of trap 58. These gases, however, will be substantiallyremoved by the diffusion pump. Any gases which are not removed by thediifusion pump must travel the length of the inner chamber through thespace defined by the inner chamber wall and intermediate chamber wall 18in order to enter the inner chamber. These gases will be condensed andheld by the cold walls of the chambers. Heat transfer to the innerchamber which may occur through the tubular section 44 is restricted bycooling the tubular section with liquid nitrogen. Also any gas leak fromthe cooled double 0 ring seal formed by flanges 52 and 38 willpreferentially enter the transition section 42 through opening 54. Thegases will then pass through openings 48 and be removed by pump 50.

While the invention has been described with respect to certainembodiments thereof, numerous modifications thereof can be made withinthe spirit of the invention. For example, the baffle 28 can beconstructed and arranged to be closable when the desired vacuum isattained in the inner chamber.

The specific geometric patterns shown and mentioned are not meant to beused in a limiting sense.

While the invention has been described with respect to the use ofgaseous helium and liquid nitrogen, other cryogenic liquids and/ orgases which provide the desired cooling can be used.

Since certain changes may be made in the above apparatus and processwithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description, or shown inthe accompanying drawings, shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. An ultra high vacuum facility comprising a first wall defining aninner chamber to be maintained at a pressure below about 10- torr, firstrefrigerating means for maintaining a majority of said first wall at atemperature below about 20 K., a second wall outside of said first wallfor defining an intermediate chamber spaced from said inner chamber,said intermediate chamber being isolated from the atmosphere by at leasttwo sealed walls throughout its whole extent except for pumping ports, asecond refrigerating means for maintaining a majority of said secondwall at a temperature below about 100 K., means positioned adjacent oneend of said inner chamber for evacuating gases from said intermediatechamber to maintain said intermediate chamber at a pressure on the orderof 10- torr, said inner chamber being in communication with theintermediate chamber at the opposite end of said inner chamber, a thirdwall outside of said second wall and defining an outer chamber spacedfrom said intermediate chamber and means for evacuating gases from saidouter chamber.

2. An ultra high vacuum facility comprising a first wall defining aninner chamber to be maintained at a pressure below about 10 torr, firstrefrigerating means for maintaining a majority of said first wall at atemperature between about 4 K. and 10 K., a second wall outside of saidfirst wall for defining an intermediate chamber spaced from said innerchamber, said intermediate chamber being isolated from the atmosphere byat least two sealed walls throughout its whole extent except for pumpingports, a second refrigerating means for maintaining a majority of saidsecond wall at a temperature on the order of 77 K., means positionedadjacent one end of said inner chamber for evacuating gases from saidintermediate chamber to maintain said intermediate chamber at a pressureon the order of 10' torr, said inner chamber being in communication withthe intermediate chamber at the opposite end of said inner chamber, athird wall outside f G of said second wall and defining an outer chamberspaced from said intermediate chamber and means for evacuating gasesfrom said outer chamber.

3. An ultra high vacuum facility comprising a first wall defining aninner chamber to be maintained at a pressure below about l0 torr, firstrefrigerating means for maintaining a majority of said first wall at atemperature between about 4 K. and 10 K., said first refrigerating meansincluding means for providing a supply of gaseous helium in coolingrelationship with the surface of said inner chamber, a second walloutside of said first wall for defining an intermediate chamber spacedfrom said inner chamber, said intermediate chamber being isolated fromthe atmosphere by at least two sealed walls throughout its whole extentexcept for pumping ports, a second refrigerating means for maintaining amajority of said second wall at a temperature on the order of 77 K.,said second refrigerating means including means for providing a supplyof liquid nitrogen in cooling relationship with the surface of saidintermediate chamber, means positioned adjacent one end of said innerchamber for evacuating gases from said intermediate chamber to maintainsaid intermediate chamber at a pressure on the order of 10 torr, saidinner chamber including an opening providing communication with theintermediate chamber at the opposite end of said inner chamber, a bafflestructure positioned adjacent said opening to prevent heat radiationfrom traversing said opening, a third wall outside of said second walland defining an outer chamber spaced from said intermediate chamber andmeans for evacuating gases from said outer chamber.

4. An ultra high vacuum facility comprising a first wall defining aninner chamber to be maintained at a pressure below about 10- torr, firstrefrigerating means for maintaining a majority of said first wall at atemperature below about 20 K, a second wall outside of said first wallfor defining an intermediate chamber spaced from said inner chamber,said intermediate chamber being isolated from the atmosphere by at leasttwo sealed walls throughout its whole extent except for pumping ports, asecond refrigerating means for maintaining a majority of said secondwall at a temperature below about K, means positioned adjacent, one endof said inner chamber for evacuating gases from said intermediatechamber to maintain said intermediate chamber at a pressure on the orderof 10- torr, said inner chamber being in communication with theintermediate chamber at the opposite end of said inner chamber, a thirdwall outside of said second wall and defining an outer chamber spacedfrom said intermediate chamber, means for evacuating gases from saidouter chamber, a first seal means for sealing said outer chamber to saidintermediate chamber, an end member forming a vacuum tight chamberadjacent said intermediate chamber, a second seal means adjacent saidfirst seal for sealing said end member to said intermediate chamber, aportion of said end member extending into said intermediate chamber andproviding support for said inner chamber and means for evacuating saidend member to a pressure on the order of 10 torr.

5. The apparatus of claim 2 wherein means are included for heating thewalls of said inner chamber and intermediate chamber to an elevatedtemperature to remove adsorbed gases.

6. The apparatus of claim 2 wherein a thermal insulation material ispositioned within the space between said intermediate chamber and saidouter chamber to provide resistance to heat transfer from said outerchamber to said intermediate chamber.

References Cited in the file of this patent

1. AN ULTRA HIGH VACUUM FACILITY COMPRISING A FIRST WALL DEFINING ANINNER CHAMBER TO BE MAINTAINED AT A PRESSURE BELOW ABOUT 10**-10 TORR,FIRST REFRIGERATING MEANS FOR MAINTAINING A MAJORITY OF SAID FIRST WALLAT A TEMPERATURE BELOW ABOUT 20* K., A SECOND WALL OUTSIDE OF SAID FIRSTWALL FOR DEFINING AN INTERMEDIATE CHAMBER SPACED FROM SAID INNERCHAMBER, SAID INTERMEDIATE CHAMBER BEING ISOLATED FROM THE ATMOSPHERE BYAT LEAST TWO SEALED WALLS THROUGHOUT ITS WHOLE EXTENT FOR PUMPING PORTS,A SECOND REFRIGERATING MEANS FOR MAINTAINING A MAJORITY OF SAID SECONDWALL AT A TEMPERATURE BELOW ABOUT 100* K., MEANS POSITIONED ADJACENT ONEEND OF SAID INNER CHAMBER FOR EVACUATING GASES FROM SAID INTERMEDIATECHAMBER TO MAINTAIN SAID INTERMEDIATE CHAMBER AT A PRESSURE ON THE ORDEROF 10**-10 TORR, SAID INNER CHAMBER BEING IN COMMUNICATION WITH THEINTERMEDIATE CHAMBER AT THE OPPOSITE END OF SAID INNER CHAMBER, A THIRDWALL OUTSIDE OF SAID SECOND WALL AND DEFINING AN OUTER CHAMBER SPACEDFROM SAID INTERMEDIATE CHAMBER AND MEANS FOR EVACUATING GASES FROM SAIDOUTER CHAMBER.